use netmgr for dispatch

Convert the dns/dispatch module (and its callers, dns/request and dns/resolver) to use the network manager.

This involves, first, refactoring dispatch to remove code that's no longer being used; moving all the calls to the isc_socket API into dispatch.c and adding functions such as dns_dispatch_connect() and dns_dispatch_send() to call them instead; replacing the isc_socket calls with isc_nm calls and then revising the dispatch API as needed to work with the new architecture.

When this is done, there will be only one remaining use of isc_socket in named - the route socket that's used for interface scanning. This will also complete the netmgr conversion of delv, mdig, and nsupdate.